1. Field of the Invention
The present invention relates to packaging machines for closing packaging trays or the like after filling, by welding a thermoplastic film on the edges of said trays, the welding operation being performed in a vacuum or in a neutral gas atmosphere. These machines are employed for packaging a wide range of food products such as ready-cooked dishes or delicatessen products, for example.
2. Description of the Prior Art
In machines of this type, the packaging trays arrive one by one at one end of the corresponding machine in order to be subsequently conveyed one behind the other to a position opposite to the filling station, then to the station for welding the closure film, and finally to the station for discharging the closed trays. At the present time, packaging trays are conveyed between these various stations by several distinct series of different driving or transfer devices. In the majority of instances, a band conveyor or the like is provided at the input of the machine for conveying the packaging trays one behind the other to a point located opposite to the filling station, then to a point located upstream of the welding station. Each packaging tray is then transported to the welding station by lateral transfer tongs which move back after positioning of a tray within the welding station. Extraction of each tray from the welding station is also carried out by other lateral transfer tongs which carry out a back-and-forth movement. Finally, discharge is carried out by means of a band conveyor or the like.
As can readily be understood, the use of several distinct series of driving or transfer devices results in a certain complexity of the machines considered as well as a relatively high cost price. Moreover, the use of lateral transfer tongs entails the need for perfect synchronization of the operation of these latter with that of the welding station. Furthermore, the systems of transfer by means of lateral tongs require frequent adjustment operations.
For all these reasons, it would be preferable to be able to use a single conveyor for carrying out the different displacements of the packaging trays over the entire length of the machine. However, this gives rise to considerable difficulties since the means employed for displacing the packaging trays must not interfere with the good performance of the moving parts of the welding station. In a station of this type, provision is in fact made for a gas-tight enclosure constituted by two complementary moving parts, namely an upwardly displaceable bottom tank and a top bell-housing which can be applied on this latter in order to clamp between them the edges of a packaging tray and the top closure film. In order to permit free motion of these parts, it is therefore necessary to ensure that the elements for displacing the packaging trays do not in any way encroach upon the path of displacement of the moving parts of the welding station. It is in fact for this reason that transportation of each packaging tray into the welding station and its subsequent discharge from this latter had hitherto been performed by lateral transfer tongs. However, as has already been mentioned, this solution is subject to major drawbacks.